Pressure swing adsorption (PSA) and vacuum/pressure swing adsorption (VPSA) processes generally employ an adsorbent material that selectively removes at least one component from a gas mixture. PSA and VPSA systems are widely used in air separation applications. Both axial and radial adsorbent bed designs can be employed.
PSA and VPSA processes are cyclic, with each cycle including adsorption and desorption steps. Rapid cycles are desired for increased productivity, and have been made possible by advances in adsorbent technology and improvements in bed design. Radial bed configurations, for example, provide relatively short bed lengths and often are preferred in larger gas separation plants.
To minimize adsorbent attrition and/or fluidization, and to reduce pressure drop, radial beds generally are operated at low gas velocities. To compensate, designs that maximize the overall inlet area to the bed, e.g. the outer radial bed diameter, are preferred. In combination with maintaining a relatively short bed length to be traversed by the mixture being separated, the resulting vessel design has a relatively large central void space.
Space that is not utilized results in less compact plant layouts. In addition, increased void vessel space causes losses in VPSA processes as relatively large volumes of void gas need to be evacuated, leading to longer cycle times.
In many cases, VPSA plants include two or more separate adsorption vessels. Extensive piping and valve systems often are required to connect them and to direct gas flows. Designs that include two adsorption zones or beds stacked in a common housing also exist but generally require increased vertical vessel dimensions and relatively large vessel void spaces for gathering and redirecting gas streams.
Furthermore, adsorption beds generally are connected to external gas storage tanks which need to be sited, installed and maintained and which, along with their valve and piping requirements, add to the overall capital and operating costs of VPSA plants.
Therefore, a need exists for adsorption vessels that minimize or eliminate the above-mentioned problems.
The invention generally is directed to apparatus used in gas adsorption processes. More specifically, the invention is directed to apparatus that includes a vessel, a radial adsorption bed within the vessel, and either an inner adsorption bed or a storage tank within an inner diameter of the radial adsorption bed. The inner adsorption bed is either an axial adsorption bed or an inner radial adsorption bed.
One embodiment of the invention includes a vessel, having a vessel side wall, an axial adsorption bed within the vessel and a radial adsorption bed surrounding the axial adsorption bed. An inner channel is defined by the axial bed and an inner porous wall of the radial adsorption bed, and an outer channel is defined by an outer porous wall of the radial adsorption bed and the vessel side wall. In a preferred embodiment, the inner channel is in fluid communication with the axial adsorption bed. The radial adsorption bed and the axial adsorption bed contain at least one adsorbent material.
Another embodiment of the invention includes a vessel that has an inner radial adsorption bed surrounded by an outer radial adsorption bed. An outer channel is defined between the outer porous wall of the outer radial adsorption bed and a vessel side wall. In one embodiment, a common porous wall separates the outer radial adsorption bed from the inner adsorption bed and an inner channel is defined by the inner diameter of the inner adsorption bed. Alternatively, the outer radial adsorption bed can be separated from the inner radial adsorption bed by an annular channel.
The vessel can be employed in a method of the invention for separating a product gas from a gas mixture. The method includes the steps of directing a gas mixture across the outer radial adsorption bed, thereby causing at least a portion of a gas component of the gas mixture to be adsorbed by the radial bed and directing partially purified gas from the outer radial bed to the inner adsorption bed, thereby causing further purification of partially purified gas and producing a product gas. In alternative embodiments, the inner adsorption bed is employed to separate a gas component from an independent gas stream or from the waste gas not adsorbed in the outer radial adsorption bed.
Another apparatus of the invention includes a vessel, a storage tank in the vessel and a radial adsorption bed surrounding the storage tank. An inner channel is defined by the storage tank and an inner porous wall of the radial adsorption bed and an outer channel is defined by an outer porous wall of the radial adsorption bed and a side wall of the vessel. The storage tank includes means for isolating the tank, for example a shut-off valve.
The vessel can be employed in a gas separation process during which a gas needed or generated during the process is stored in the storage tank. In one embodiment of the invention, the vessel is employed in an air separation process in which the storage tank is employed to store void gas, waste gas, or product gas. In a preferred embodiment, the method includes pressurizing with air a partially pressurized radial bed, thereby causing the radial bed to adsorb nitrogen. Oxygen that permeates the radial bed is directed to a product surge tank. The radial bed is partially depressurized, thereby releasing void gas (oxygen enriched) from the radial adsorption bed. The void gas is directed to an equalization tank. Nitrogen adsorbed in the radial adsorption bed is evacuated, after which oxygen from the product surge tank is directed to the radial adsorption bed, thereby desorbing nitrogen from the adsorption bed and purging the radial adsorption bed of nitrogen gas. Additional void gas from the equalization tank is then directed to the radial adsorption bed, thereby partially pressurizing the radial adsorption bed. The partially pressurized bed is then ready for another cycle.
The invention has numerous advantages. For example, the vessels of the invention have reduced void space, thereby minimizing the amount of void gas to be evacuated and reducing PSA or VPSA cycle time. In many cases, vessels of the invention can be fabricated inexpensively, by modifying existing vessels that already house a radial adsorption bed. The invention is compatible with compact VPSA plant designs and results in capital and operational cost reductions. By employing vessels of the invention, external storage tanks and/or separate adsorption beds can be eliminated, possibly along with some piping and valve requirements. Vessels that combine a central axial bed surrounded by a radial bed result in an increase in the overall vessel separation capacity, minimize overall vessel size and void space, and provide a simple arrangement for directing gas from one bed to the other. Locating tanks that contain flammable or explosive gases or tanks fabricated from UV sensitive materials within the central void space of a radial bed also eliminates the need for additional safety devices or UV protective tank enclosures. In addition, VPSA processes that recycle void gas and use product gas as a purge generally can provide improved product flow rate.